Conventional industrial optical lithography methods are approaching a fundamental limit at a resolution of the order of 70 nm, due to the non-availability of convenient light sources with wavelengths of less than 0.13 μm, the shortest wavelength excimer laser beams. Experimentation in the use of X-rays as an “optical” source with wavelengths below this level has been carried out for many years, but the technology is complex, expensive and difficult to work with, and commercial application does not yet seem viable. Since 100 nm resolution or less is expected to be in use in chip production in the near future, alternative approaches to nanofabrication have been examined in recent years. Techniques such as electron beam and ion beam lithography, which have higher resolution because of their short de Broglie wavelengths, though also in their infancy, are already commercially available. However, these methods too have the disadvantages that they are technologically complex and expensive to perform, and that because of the high energy of the beams, special techniques must be adopted to avoid damage to the surface material, and charging of the surface which could affect the pattern being generated.
One of the most promising schemes currently being developed, uses the methodology of atom optics, in which beams of neutral atomic particles are manipulated and focused by the lensing effect of interfering laser fields, and thus directly deposited onto a substrate, to form extraordinarily small features. Currently this technique is able to generate details of down to several tens of nanometers. The technique was originally developed by a joint group from AT&T Bell Laboratories and Harvard University, as described in their first publication on the subject in an article entitled “Using Light as a Lens for Submicron, Neutral Atom Lithography” by G. Timp et al., published in Physical Review Letters, Vol. 69, pp.1630-1639, 1992, and concurrently by researchers from NIST, as described in U.S. Pat. No. 5,360,764, for “Method of fabricating laser controlled nanolithography” to R. J. Celotta et al. Both of these documents are herewith incorporated by reference, each in its entirety. As an alternative to direct writing, the atoms have been used to expose a resist, even by focused neutral atoms at thermal energies. The energy for the resist modification comes from the internal energy of excited atoms, as described in U.S. Pat. No. 5,851,725 for “Exposure of lithographic resists by metastable rare gas atoms” to J. McClelland.
Atomic deposition techniques using light beams for focusing the atomic beam have several advantages over the currently available non-optical high-resolution techniques mentioned above, such as electron beam and ion beam lithography. It does not damage the deposition surface, it avoids charging the surface since slow neutral atoms are used, there are no limits set by charged-particle interactions in the beam itself, and contamination of the samples is prevented in the resist-free, direct deposition mode. Theoretically, the resolution is limited only by the quantum mechanical deBroglie wavelength of a beam of thermal atoms which is typically about 10 pm. This is smaller than the spacing between atoms, such that it should theoretically be possible to grow structures as small as a single atom. Currently, a number of academic and technological laboratories worldwide are active in developing this technology.
However, the currently developed laser-focused deposition methods suffer from a major disadvantage in that there exists a considerable level of background deposition which limits the sharpness of the nanostructures deposited, and causes overlap between written features. In addition to the well-known phenomenon of surface migration of deposited atoms, there are two main reasons for this background. Firstly, the sinusoidal standing light wave accurately focuses the atomic beam only for atoms passing near the minimum of the light-induced potential, where its profile is parabolic. In this respect, a sinusoidal standing wave behaves like a conventional optical lens with a considerable level of aberrations, in particular, “spherical” aberration. The solutions known for correcting such aberrations in conventional optical elements, such as the use of aspheric or free-form surfaces, cannot be easily applied in the case of atom optics. In order to improve the quality of the focusing by means of a reduction of the background level, a combination of a standing light wave with a nano-fabricated mechanical mask with multiple slits has been suggested by S. Meneghini, et al., in an article entitled “Atomic focusing and near field imaging: A combination for producing small-period nanostructures”, published in Applied Physics B, Vol. 70 , pp. 675-682 (2000). The slit structure blocks the atoms passing far from the potential minimums. However, this complicates the set-up considerably, it being difficult to line up the slits and field nodes with sufficient mechanical accuracy, and significantly limits the deposition rate, since only those atoms passing through the slits are available for the deposition process.
Secondly, because of the longitudinal velocity spread of the incident atomic beam the atom lens suffers from an effect which is the equivalent of chromatic aberration in conventional optics. In general, thermal atomic beams have significant transverse and longitudinal velocity spreads. This can be reduced by techniques such as laser cooling and mechanical collimation. However, these techniques still cannot produce ideally monochromatic beams, so that the focus is still degraded because of this effort.
There therefore exists an important need for a method and apparatus for performing atomic and molecular beam focusing and deposition, which overcome the disadvantages and drawbacks of previous methods and apparatus.
The disclosures of all publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety.